A very sensitive and stable delta pH meter was developed allowing the measurement of delta pH in the range of a few ten-thousandths of a pH unit per minute. With this meter we have learned that succinoxidase vesicles prepared by a variety of techniques are capable of net proton ejection with about 10 percent the H ion/0 ratio as intact membranes. The principal reasons why the measured ratio is not higher appear to include an impairment in substrate transport, membranes that are leaky to protons, and extent of true asymmetric reconstitution. Attempts are underway to obtain reconstituted membranes with maximal H ion/0 ratios.